Field of the Invention
The present invention relates to methods of manufacture of products such as semiconductor devices using lithographic techniques.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Multiple layers, each having a particular pattern and material composition, are applied to define functional devices and interconnections of the finished product.
Current and next generation processes often rely on so-called multiple patterning techniques to produce device features having dimensions far smaller than can be printed directly by the lithographic apparatus. Multiple patterning steps, each having its own mask or reticle, are performed to define a desired device pattern in a single layer on the substrate. Many different examples of multiple patterning are known. In some processes, a regular, grid structure is formed as a basis for the desired device pattern. Then using a circuit-specific mask pattern, lines that form the grid structure are cut at specific locations to separate the lines into individual segments. The grid structure may be exceptionally fine in dimensions, with a pitch in the tens or even teens of nanometers.
In a lithographic process, it is desirable frequently to make measurements of structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers of a substrate. Final performance of manufactured device depends critically on the accuracy of positioning and dimensioning of the cut mask relative to the grid structure. (The cut mask in this context is what defines the circuit-specific locations at which the grid structure is modified to form functional circuits.) Overlay error may cause cutting or other modification to occur in a wrong place. Dimensional (CD) errors may cause cuts be too large, or too small (in an extreme case, cutting a neighboring grid line by mistake, or failing to cut the intended grid line completely).
Other performance parameters of the lithographic process may be also of interest, for example in optical lithography parameters of focus and exposure dose may also require measuring.
However, the dimensions of modern product structures are so small that they cannot be imaged by optical metrology techniques. Small features include for example those formed by multiple patterning processes, and pitch-multiplication. (These terms are explained further below.) In effect, the structures are too small for traditional metrology techniques which cannot “see” them. Hence, targets used for high-volume metrology often use features that are much larger than the products whose overlay errors or critical dimensions are the property of interest.
While scanning electron microscopes are able to resolve modern products structures, measurements performed with scanning electron microscopes are much more time consuming, as well as more expensive, than optical measurements.